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with
Chris Halliday
the grid. Long low-voltage runs to an
installation will cause voltage rise, as the
generated current must push out through
the cable impedance. Effects may be
localised to the installation yet they may be
transferred to neighbours, depending on
the size of cables and generator, and the
grid layout.
The Wiring Rules are generally silent
on protection for sustained overvoltage.
However, there are some generic clauses:
>
1.6.1 requires the installation design to
function properly and safely;
>
1.7.1 stipulates that electrical equipment
be selected and installed to operate
safely; and,
>
1.6.4 deals with the utilisation voltage of
the installation.
IMPULSIVE TRANSIENTS
The Wiring Rules cover events caused by
lightning or switching operations/
These events are labelled as overvoltage
but power-quality people generally call
them ‘impulsive transients’.
Lay people talk about surges and surge
protection, but these terms have no real
technical basis. We will use the correct
terminology of impulsive transients.
Part 1 of the Wiring Rules specifies at
Clause 1.5.11.3 that protection for impulsive
transients is not a requirement, but
measures are stated that can be used if an
electrical installation is protected against
‘overvoltage’ (see Clause 2.7).
Clause 2.7 also discusses overvoltage
protection measures for an insulation fault
between the electrical installation and a
higher-voltage circuit and also for resonant
phenomena. What the?!
Well, the former requires adequate
insulation screening – or segregation of
circuits and transformers with adequate
insulation, screening or separation of
windings for transformers.
For the latter, the Rules are silent.
Resonance occurs when the reactance
of capacitors equals that for inductors,
and transient or sustained overvoltages
can occur at a particular frequency. Most
things have a resonant frequency but
resonance won’t be an issue unless that
frequency occurs.
What does this mean for a local sparkie?
Probably not much, unless you are involved
in industrial plants using capacitors and/or
high-voltage transformers. This would be a
complete topic in itself.
Although the Rules do not require
impulsive transient protection, guidance is
given at Appendix F on how to select and
install the right protection if required.
You would generally consider impulsive
transient protection:
>
if lightning is prevalent;
>
in outer-suburban or rural areas,
including exposed sites perhaps on a
hill or in an open paddock;
>
if the installation is connected via long
overhead powerlines; and,
>
if sensitive electronic equipment is used.
UNBALANCE
Protection for voltage or current
unbalance (also called imbalance) is not
mentioned in the Wiring Rules, but this
doesn’t mean it is unimportant.
Voltage unbalance is regarded as any
differences in the three-phase voltage
magnitudes and/or a shift in the phase
separation of the phases from 120º. It is
generally expressed in percentage terms,
and there are simple – and quite complex –
methods for calculating it.
EMONA
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